Pressure sensors may be used in cars, for example, to measure oil pressure, to measure tire pressure, and to measure brake fluid pressure. Additionally, there are numerous non-automotive applications for pressure sensors.
Pressure sensors have been made using wafers by conventional micromachining methods. For example, pressure sensors have been built by thinning sensing membranes from the back of wafers. This method may be described as both reliable and mature, but the backside processing can be expensive and can consume large amounts of die area. The membrane may be formed by etching a wafer from the back until it is a predetermined distance from a front surface of the wafer and then stopping the etch. The distance from the front face may be tens of microns, and may be up to a hundred microns. Sensors, which are often piezoresistors, can be arranged in the top of the silicon that bridges the hole in order to measure the deflection of the silicon as a function of the pressure that is exerted across it. One problem with this technique is that cutting a hole from the back of the wafer can consume a lot of space on the die. An etching technique often used in this application is an anisotropic silicon etch, for instance a KOH etch, which generally does not cut completely vertical holes. The holes are usually pyramidal, and therefore require a die that is a few millimeters across on the bottom in order to make a hole that is a few hundred microns up to a half millimeter across on the top of the wafer. The larger die size increases the cost of the pressure sensor.
The strength of the membrane in terms of deflection may be approximately proportional to the cube of the thickness of the membrane. Therefore, a 10% error in the thickness may cause about a 30% error in the strength of the membrane, which may lead to about a 30% error in the sensitivity of the pressure sensing device using the membrane. In other words, a small error in the thickness may lead to a large error in the pressure measurement. The stiffness in terms of deflection is approximately proportional to the size (i.e. the lateral dimension) to the fourth power. The pyramidal pits caused by KOH etches have sloped edges in which the peak of the pyramid is at the back of the membrane. If the wafer varies in thickness, the size of the tip of the pyramid underneath the membrane can vary as well. Therefore, the size (i.e. the diameter) and the thickness of the membrane can be hard to control and may therefore impair the accuracy of the pressure sensor constructed in accordance with conventional methods.
Techniques have been developed for building pressure sensors entirely from the front of wafers by forming porous silicon areas on the top of the wafers, covering them with single crystal membranes of epitaxial silicon and melting away the buried porous silicon, thereby creating sealed evacuated cavities. Piezoresistors on the top of the silicon over these cavities sense the membrane's deflections. This technique may be limited, however, by crystal defects in the epitaxial silicon grown over the porous silicon.